Blender equivalent of GIMP Levels tool
The upper part of the Levels tool, "Input Levels",



should be equivalent to mapping the "Value" channel of the input image to the output image through a ColorRamp:



Separate HSVA, then recombine them with Combine HSVA, but add a Color Ramp between the V sockets. In the Color Ramp add a slider ("+") and moving the three sliders should behave the same as in Levels.